While plastic materials have been used for a number of years in the manufacture of various types of optical elements, including eyeglass lenses, telescope and camera lenses, and the optical components of windows, canopies, reflectors, and the like, and although such plastics are clearly superior to glass in terms of strength-to-weight ratios and their resistance against fracture, they also have the well-recognized disadvantage of being less abrasion resistant than glass. Accordingly, considerable effort has been directed in recent years to improving the abrasion resistance of optical elements formed from polycarbonates and other plastic materials.
Various plastic materials have been used in the past in fabricating optical elements, such materials including polycarbonates, polyacrylics, cellulose acetate, polystyrene, polysulfones, cellulose acetate butyrate, polyesters, and selected copolymers thereof. At present, most opthalmic eyeglass lenses are formed of a particular polycarbonate(poly(diethylene glycol bis allyl carbonate)) marketed under the designation "CR-39" by PPG Industries, Pittsburgh, Pennsylvania, because, among other things, that material has greater abrasion resistance than other plastic materials. In comparison with glass, however, CR-39 is still far less resistant to abrasion and, as a result, considerable effort has been expended in the optical field to develop some treatment for CR-39, and for other rigid transparent plastics, to make the surfaces of such materials harder, with abrasion-resistant properties more akin to those of glass. The following U.S. patents are illustrative of such efforts: U.S. Pat. Nos. 3,645,779 (vacuum-depositing a glass coating upon a transparent polymeric substrate to form an optical element having increased abrasion resistance); 3,019,131 (coating plastic optical elements with polymerizable derivatives of polyalkyleneglycol to increase mar resistance); 3,811,753 (applying vitreous silicate coatings to plastic optical elements to improve abrasion resistance); 2,904,450 (coating plastic optical elements with germanium and silicon oxide to produce abrasion resistance similar to glass); 2,851,388 (cladding a plastic layer with one or more glass or hard plastic layers to improve abrasion resistance); 2,361,589 (coating a plastic opthalmic lens with a layer of a harder material such as glass to increase abrasion resistance). While such techniques for forming a glass or glass-like coating on plastic optical elements have been encouraging, the technical difficulties and operating costs are sufficiently great that no substantial commercial success has yet been achieved.